The present invention concerns a stove for solid fuel, typically for the heating of one or more living rooms and using wood and/or briquettes as fuel. Such stoves can be configured with different shapes for more or less regulation of the air supply, whereby distinction is made between the following kinds:
A: Primary air: Air which from a lower intake opening is fed to the area down under the stove""s fire grating which supports the fuel over an underlying ash drawer, said air thus being drawn up directly through the fuel pile upon the effect of an arising or commenced combustion and related heat development in the fuel, respectively by a chimney draft established hereby with the view to strong feeding of the fire.
B: Secondary air: Air which is supplied to the pile of fuel over the fire grating, preferably after passage of a channel system in the stove for the pre-heating of this air, with the view to moderate feeding of the fire, and
C: Tertiary air: Air which is supplied to the uppermost of the combustion chamber, with the view to supplying oxygen for ensuring that remaining combustible gases are burned off, partly for maximum utilisation of the fuel by clean combustion and partly to preclude danger of explosion.
The respective air intake openings are normally provided with dampers which can be regulated manually, and which following instructions the user can operate in an appropriate manner in various operational phases of the stove, but it is also known, however, to let one or more of these dampers be controlled automatically, based on a bimetallic sensing of the combustion temperature, i.e. with the view to achieving an even combustion with the desired intensity.
However, it is also known to use an electrical servo-control of the regulation dampers for intake air, cf. EP-A-0 604 388, depending on a sensing of the temperature of the flue gas, and also depending on a measurement by means of a lambda sensor of the CO contents of the gas. No distinction is made here between different air supplies or different operational situations. When or after a stove burns out, a total closing of the regulation damper for intake air is effected, among other things to prevent flue gas explosion, and thereafter the damper is opened depending on a sensed opening of the stove door for renewed starting of the stove.
U.S. Pat. No. 4,556,044 discloses a stove with inlets for both primary and secondary air, and with a damper which can be switched over between expedient air distributions with respectively xe2x80x9chigh firexe2x80x9d, xe2x80x9cnormal combustionxe2x80x9d and xe2x80x9clow combustionxe2x80x9d. Use is made of a damper which by being switched over serves the two air intakes simultaneously.
However, it is hereby characteristic that the relevant automatic control refers precisely to the operative normal combustion in the stove, and not at all to the conditions which arise in respectively a lighting-up, a re-firing and a burning-out sequence, where the ideal air controls are quite different from the conditions during the normal combustion. It is precisely for this reason that the user should be informed how the dampers should stand during these special phases, but even with this knowledge it will be almost impossible for the user to operate the dampers in an optimum manner, when the control parameters comprise limit values for the combustion temperature and for the content of oxygen in the flue gas.
In the light of the above, with the invention there has been taken the special step of introducing an overriding, electronic and individually-directed control of the damper positions by a programmed control unit with keying-in function, which makes it possible for the user to inform the control unit that there has now been initiated a lighting-up or a re-firing phase, after which by means of sensors connected for the temperature of and possibly the oxygen content of the flue gas, the control unit will be able to control the air supplies in an optimum manner, also during the lighting-up, the re-firing and burning-out phases.
In practice, this will manifest itself by use being made of regulation dampers which in a mutually independent manner are driven by controllable step-motors, and that in connection with the stove installation there is provided an operation box which, e.g. has keying-in means in the form of pushbuttons, enables the user to key-in the time for or the event of an actual lighting-up of the oven. The same operation box can have keys for entering a desired working temperature of the stove, preferably merely for either xe2x80x9chighxe2x80x9d or xe2x80x9clow effectxe2x80x9d, and possibly have a display which confirms to the user that the oven is now operating under xe2x80x9clighting-upxe2x80x9dconditions or normal operation, or possibly in the burning-out condition, the latter as a signal to the user that the stove must be fed with new fuel if it is desirable to maintain the burning function. This information can possibly be supported by the emission of an acoustic alarm signal which, however, the user ought to be able to deactivate, e.g. if the oven with adjusted related air control is desired to burn out after bedtime. On the other hand, if the user chooses to add new fuel, the control unit can be fed with this information by a simple entry via the keypad, after which the air control undergoes a radical change for a favourable ignition of the new fuel.
The overriding control parameter will naturally be the temperature, which is best measured with a sensor placed in the flue gas discharge pipe, preferably 15-20 cm up in the flue gas discharge pipe. The user can key-in a desired combustion temperature, e.g. of 300xc2x0 or 400xc2x0, corresponding respectively to said xe2x80x9clowxe2x80x9d and xe2x80x9chighxe2x80x9d effect, and if or when it is ascertained via the sensor that the temperature is lower than the desired value, the control must then be directed in very different ways, depending on whether this is the result of a lighting-up phase or a random reduction in connection with an already-established combustion sequence. When lighting-up, there should thus be established a full air supply, while with an operative temperature reduction there should only be carried out a graduated or selective alteration of perhaps only one of the air intakes. In its way, it will be relatively easy to programme the control unit in such a manner that it can automatically detect whether the one or the other situation arises, since via the sensor it will, of course, be able to be registered whether there has previously arisen a more or less high operating temperature or whether there occurs an increase in a very low ignition temperature, and on this basis the lighting-up can possibly be registered in a fully automatic manner. However, it will give an even more sure control signal if the user indicates a lighting-up by a keying-in signal.
When lighting-up and with normal working operation, it is the said primary and secondary air supplies which are in focus, controlled only by the flue gas temperature. During the lighting-up phase, the primary air damper should be held completely open for approx. 10 minutes, also after the temperature of the flue gas has reached up to its set value of e.g. 300 or 400xc2x0, in that this damper, however, can then be controlled for a limited opening of e.g. 10-20% with the object of establishing a warming-through of the stove. After these approx. ten minutes, and after the desired operating temperature has been reached, the control unit can bring about a total closing for the flow of primary air. This applies also to operative conditions as well as with burning out.
The damper for the said secondary air must be controlled in such a way that it can not be totally closed so long as a combustion can at all take place in the stove, in that the secondary air will be responsible for the maintaining of a minimal combustion, also during a burning-out phase when the primary air is shut off, and a small intake of air will preclude the risk of explosion. During the lighting-up phase, the supply of secondary air must be fully open, while after a warming-through has been achieved, e.g. after the said 10 minutes, a change is made to actual regulation operation precisely with the help of the secondary air. If, during operation, a change is made from xe2x80x9chigh effectxe2x80x9d (400xc2x0) to xe2x80x9clow effectxe2x80x9d (300xc2x0), a down-regulation is effected, preferably so that the stove is controlled down in steps of e.g. 10xc2x0 per minute, which will provide a more or less even fall in the temperature.
In the event of xe2x80x9cfuel neededxe2x80x9d or during the start of a burning-out phase, e.g. defined by a temperature interval between 300xc2x0 and 230xc2x0, the secondary air should be fully opened for optimum utilisation of the fuel, so that this air can have as good an ignition effect as possible on the newly added fuel, while with a definite burning-out, e.g. defined by the temperature range between 230xc2x0 and 50xc2x0, there can be throttled down to an only slightly open supply of secondary air. When the stove has gone out (T less than 50xc2x0), the damper should be closed completely.
The object of the tertiary air is to ensure a clean combustion, i.e. with low emission of carbon monoxide and other combustible gases. In an indirect, but reasonably reliable and inexpensive manner, this can be monitored by using an oxygen flow-meter of the lambda probe type, in that for example it has been found that combustion is clean when, at a flue gas temperature of 400xc2x0, there appears an oxygen content in the flue gas of more than 9%, while the corresponding value at 300xc2x0 is 12%. If the oxygen content is greater or smaller, a regulation of the tertiary air must be made respectively up or down. During the lighting-up, the air supply must be at maximum, while at xe2x80x9cfuel neededxe2x80x9d or the start of a burning-out (230xc2x0 less than T less than 300xc2x0), operation can take place with a requirement control based on the information from the lambda probe. When the stove has gone out completely and with a cold stove, the supply should be closed.